Electrical signals cause a heart to beat. In a healthy patient, regular heart beats pump blood through the cardiovascular system. The human cardiovascular system is responsible for receiving oxygen-deprived blood into the heart from the venous system of the body, delivering the oxygen-deprived blood to the lungs to be replenished with oxygen, receiving the oxygenated blood from the lungs back into the heart, and delivering the oxygenated blood to the body via the arterial vasculature. This process is regulated within the heart by electrical pulses that control the operation of the heart's receiving and pumping chambers.
In a healthy heart, the sinoatrial node of the heart generates electrical pulses in a consistent and regulated fashion to regulate receiving and pumping blood in the heart's chambers. The electrical impulses propagate as activation wavefronts across the atria, the upper chambers of the heart, and cause cells of the atria to depolarize and contract, which forces blood from the atria to the ventricles, the lower chambers of the heart. The ventricles receive the blood from the atria, and the wavefront, after passing through the atrioventricular node and moving to the Purkinje system, moves to cells of the ventricles causing the ventricles to contract and pump the blood to the lungs and to the rest of the body.
Various aspects of cardiac activity (e.g., heart rate, arrhythmias) can be detected by measuring, recording, and analyzing cardiac electrical signals, such as an electrocardiogram (ECG) signal. One way of measuring ECG signals involves attaching electrodes, typically ten, externally to a patient's skin and sensing the electrical signals that form the ECG waveform. Implantable monitoring systems can be implanted under the skin with electrodes that sense subcutaneous electrical signals, including ECG signals, which are analyzed as being indicative of cardiac activity.
Heart rate variability can be an important factor to consider when processing or otherwise using a cardiac signal. It is, in some instances, a measure of patient health and cardiac stability. For instance, it can be difficult to quantify how effectively a drug (such as, for instance, beta blockers) is regulating cardiac heart rate due to heart rate variability. In some instances, device-based atrial fibrillation (AF) detection involves measures of heart rate variability. Devices commonly use fixed variability ratios (i.e., 8%, 12%, or 18%); however, each patient has a unique variability that is modulated by a multitude of factors, such as age, gender, autonomic drive, disease severity, medication usage, etc. Therefore, what might be considered regular in one patient may be dangerously irregular in another.